The magnetic storage industry has been increasing the storage capacity of hard drives through technological advancements such as perpendicular recording systems and magneto-resistive (MR) heads. The MR head includes an MR element that is made of a material that changes electrical resistance in response to the strength of a surrounding magnetic field.
The MR head normally glides over the spinning magnetic disk. When the MR head hits a protruding object on the disk surface, the MR element heats up rapidly and the heat decays relatively slowly. The effect of this transient phenomenon is a change in the baseline of the read-back signal that is output by the MR head, as illustrated in FIG. 1. This baseline change contains a substantial low-frequency component, which causes loss of read-back data. The severity of the loss depends on the robustness of the data detection system and the rate at which TA events occur.
Perpendicular recording systems provide higher data storage densities than conventional recording systems. However, perpendicular recording systems are typically subject to an increased rate of TA events due to the relatively close proximity of the recording head and the magnetic media. In addition, the heating of the recording head from the TA event is higher than the heating that is experienced in longitudinal recording systems due to the closer relative distance between the recording head and the media. The increased heating translates into a transient of increased duration and amplitude in the output signal of the recording head.
Therefore, the recording head signal of perpendicular recording systems typically includes both an increased rate of TA events and TA events having a longer duration. With the increased likelihood of errors, it is possible that the error correcting code (ECC) that is used in the recording system may not be able to regenerate the user data.
In perpendicular magnetic recording, a detector that utilizes a non DC-free equalization target can be used. Non-DC-free detectors are sensitive to DC-offset (baseline offset). A baseline correction circuit is often used to minimize the DC-offset. There are different sources of DC offset in a perpendicular magnetic recording system. For example, AC coupling of the pre-amplifier to the MR head may impact DC offset. The read channel front end may act as a high pass filter and cause data dependent baseline wander. Other sources are DC-offsets that are produced by analog circuits. TA, as mentioned above, is a low-frequency transient, which can be viewed as a DC-distortion.
The baseline correction circuit may generate the baseline correction signal based on bit detector outputs. When the bit detector outputs are unreliable due to TA events, signals output by the baseline correction circuit will also be unreliable. Since TA is a transient behavior, errors at the detector output are usually limited and can be handled by an error correcting code (ECC) present in most magnetic recording systems. For longer TA events, however, the error correcting capability of the ECC is typically insufficient. Furthermore, if the TA event is not very long, but the correct baseline can not be recovered, the increased error rate at the detector output may continue after the TA event is over. The error correcting capability of the ECC may be insufficient.